The use of abrasive particles in a stream of drilling fluid to drill a wellbore has been proposed as an alternative to conventional drilling methods such as rotary drilling with a roller-cone drill bit or a PDC drill bit. In such alternative drilling method a jetting device ejects a high velocity stream of a mixture of drilling fluid and abrasive particles against the bottom of the borehole thereby deepening the borehole.
U.S. Pat. No. 3,838,742 discloses a drill string provided with a drill bit having a number of outlet nozzles. Drilling fluid containing abrasive particles is pumped via the drill string through the nozzles thereby producing high velocity jets impacting against the borehole bottom. The abrasive particles accelerate the erosion process when compared to jetting of drilling fluid only. The rock cuttings are entrained into the stream that returns to surface through the annular space between the drill string and the borehole wall. After removal of the rock cuttings from the stream, the pumping cycle is repeated. However, this system has the drawback that continuous circulation of the abrasive particles through the pumping equipment and the drill string leads to accelerated wear of these components. Another drawback of the known system is that constraints are imposed on the rheological properties of the drilling fluid, for example a relatively high viscosity is required for the fluid to transport the abrasive particles upwardly through the annular space.
European patent 1175546 discloses a drill string provided with a drill bit having a plurality of outlet nozzles through which a mixture of drilling fluid and abrasive particles is ejected against the borehole bottom. The lower part of the drill string is provided with a recirculation assembly for re-circulating the abrasive particles in the lower portion of the borehole. The re-circulation system catches the abrasive particles as these flow upwards through the annular space between the drill string and the borehole wall, and re-circulates the abrasive particles through the lower end part of the drill string and the outlet nozzles. Damage to the pumps and the upper part of the drill string due to contact with the abrasive particles is thereby substantially prevented.
However it was found that a minor portion of the abrasive particles bypasses the recirculation system and flows upwardly to surface through the annular space. If the loss of abrasive particles is not compensated, a decreasing amount of abrasive particles remains available for deepening the borehole. It also was found that compensating for the loss of particles by feeding low amounts of particles into the stream at surface via a feed device having a narrow flow opening, potentially leads to blocking of the narrow flow opening with abrasive particles.
It is therefore an object of the invention to provide an improved device for feeding particles into a stream, which device overcomes the drawback of the prior art.